Soil itself is hydrophilic, but when it becomes “waterproof,” it is scientifically described as exhibiting “water repellency.” This is usually not a good thing, as it affects water infiltration and plant growth. Soil water repellency, or hydrophobicity, is a complex soil physicochemical phenomenon. It does not mean that the soil completely repels water, but rather that the contact angle of water on the soil surface or within the soil is greater than 90 degrees, making it difficult for water to spontaneously wet and infiltrate. Its essence is the transformation of the surface energy of soil solid particles from a high-energy hydrophilic state to a low-energy hydrophobic state. This transformation mainly stems from interactions at several levels.
How does soil become water-repellent?
Soil becomes water-repellent due to the presence of hydrophobic substances. Hydrophobic substances are organic molecules that repel water. Microbial activity, organic matter, and the decomposition of plant tissues all release hydrophobic substances into the soil. These hydrophobic substances form a thin, waxy coating around each soil particle, making the soil hydrophobic (water-repellent). The higher the hydrophobicity of the soil, the lower the water infiltration rate. Water molecules are bipolar and have strong cohesive forces: they attract similar molecules. The strong attraction between water molecules and their weak ability to bind to the waxy soil particles results in water droplets forming with a high contact angle and surface tension. This high surface tension prevents the water droplets from spreading over a large surface area. Whether soil is water-repellent depends not only on the presence of hydrophobic substances but also on the soil texture. Coarse-textured, sandy soils with less than 5% clay content are very susceptible to becoming water-repellent.
- Decomposition products of organic matter: This is the most common cause. The fallen leaves of certain plants (such as pine trees and eucalyptus), root exudates, or microorganisms (such as certain fungi) release long-chain aliphatic organic compounds (such as waxes, resins, and organic acids) during decomposition. These substances coat the surface of soil particles (especially sand grains), forming a hydrophobic film that changes the soil particles from hydrophilic to hydrophobic.
- Microbial activity: The hyphae of certain types of soil fungi (such as some basidiomycetes) secrete hydrophobic substances, which can also lead to localized or widespread water repellency in the soil。
- Drying process: Many water-repellent soils do not exhibit water repellency when wet, but when they are thoroughly dried, the water repellency becomes very strong. Re-wetting requires considerable effort (such as prolonged rainfall or human intervention). This is because the drying process causes the hydrophobic organic substances to arrange themselves more tightly and adsorb onto the surface of the soil particles.
In this case, additional irrigation will only increase water, labor, and pumping costs, with little effect on improving soil moisture. Even slight soil hydrophobicity can negatively affect water movement in the soil, subsequently impacting plant growth and development, ultimately leading to reduced crop yields and lower product quality.
How do soil wetting agents work?
Soil wetting agents are agricultural chemicals specifically designed to overcome soil water repellency. Their scientific name is “non-irrigating wetting agent” or “soil penetrant.” The vast majority of them are surfactants, and their action is not a simple “lubrication,” but rather a series of precise physicochemical processes.
1. Core Mechanism: The Amphiphilic Structure-Activity Relationship of Surfactants
All wetting agent molecules share a common structural feature: one end is a hydrophilic group (such as a polyoxyethylene chain, sulfonate group, or carboxyl group), and the other end is a hydrophobic (lipophilic) group (usually a long-chain alkane). It is this “dual nature” that makes them key to solving the problem of water repellency.
- Reducing Surface Tension: Water has a high surface tension (72.8 mN/m at 25°C), which causes water droplets to tend to maintain a spherical shape, minimizing contact area with hydrophobic surfaces. When wetting agent molecules are added, they quickly accumulate at the water-air interface, with their hydrophobic tails extending into the air and their hydrophilic heads remaining in the water, significantly reducing the surface tension of the water (to below 30 mN/m). With reduced tension, water droplets are more easily deformed, spread, and penetrate.
- Reducing Solid-Liquid Interfacial Tension: This is a more crucial step. Wetting agent molecules can orient themselves at the soil particle (hydrophobic surface)-water interface. Their hydrophobic tails are firmly adsorbed to the surface of soil particles coated with organic “waxy” substances through van der Waals forces, while their hydrophilic heads extend into the water. This is equivalent to building a “molecular bridge” between the hydrophobic soil particles and the polar water molecules, replacing the originally high-energy “water-hydrophobic surface” interface with two low-energy interfaces: “water-hydrophilic head” and “hydrophobic tail-hydrophobic surface.”
2. Dynamic Stages of the Action Process:
- Stage One: Adsorption and Initial Wetting: When an aqueous solution containing a wetting agent comes into contact with hydrophobic soil, the wetting agent molecules, due to their hydrophobic ends, preferentially adsorb onto the hydrophobic sites of the soil particles before water molecules. This instantly changes the surface properties of these sites, transforming them from hydrophobic to hydrophilic, allowing water molecules to attach. The establishment of this “beachhead” is the first step in the initiation of water infiltration.
- Stage Two: Penetration and Displacement: As more solution arrives, wetting agent molecules continue to adsorb at the leading edge of the advancing wetting front, continuously expanding the hydrophilic area. Simultaneously, some wetting agents can also exert a certain solubilizing effect, where their micelles can encapsulate trace amounts of hydrophobic organic matter, causing it to detach from the soil surface and disperse into the water. However, this is not the primary mechanism and may lead to the migration of organic matter.
- Stage Three: Redistribution and Sustained Action: After water infiltration, a portion of the wetting agent molecules will firmly adsorb onto the soil particles, providing lasting hydrophilic improvement. Another portion moves with the water, continuing to act on hydrophobic sites in deeper or drier areas. High-quality wetting agents can maintain their effectiveness even after multiple wetting and drying cycles.
3. Classification and Selection of Wetting Agents:
Anionic type: such as Rhamnolipid and Xanthan gum. They are easily adsorbed by positively charged sites in the soil and are more effective in sandy soils with low clay content. However, they may react with polyvalent cations (Ca²⁺) to form precipitates and become ineffective.
Non-ionic type: such as Tea Saponin and Silicone Surfactant. They are insensitive to electrolytes, stable in hard water and at different pH values, have moderate adsorption characteristics, effectively reduce surface tension, and have low phytotoxicity. Their hydrophilic-lipophilic balance (HLB) value is key to selection; those used for soil wetting typically require a higher HLB value (>13).
Amphoteric and bio-based wetting agents: such as Soya Bean Lecithin. They have mild performance but are more expensive. The emerging trend is the development of biodegradable wetting agents based on plant extracts or microbial fermentation products.
In summary, soil water repellency is a unique outcome of the interaction between the carbon cycle and soil mineral surfaces in nature, disrupting the fundamental function of soil as a “water reservoir.” Soil wetting agents, on the other hand, represent a clever artificial intervention that utilizes the amphiphilic properties of surfactant molecules to “reprogram” the surface properties of soil particles at the microscopic level, transforming hydrophobic interfaces into hydrophilic ones. This restores normal water flow and distribution, ensuring the sustainability of agricultural production and the health of ecosystems.